CT imaging was undertaken on the skull of ~20 Myr-old Miocene Tremacebus
harringtoni. Here we report our observations on the relative size of the olfactory fossa and its implications for the behavior of Tremacebus. The endocranial surface of Tremacebus is incomplete making precise estimate of brain-size and olfactory fossa-size imprecise. However, olfactory fossa breadth and maximum endocranial breadth measured from CT images of one catarrhine and eight
platyrrhines for which volumes of the olfactory bulb and brain are known show that
the osteological proxies give a reasonably accurate indication of relative olfactory
bulb size. Aotus has the largest relative olfactory fossa breadth and the largest olfactory bulb volume compared to brain volume among anthropoids.
Tremacebus had a much smaller olfactory fossa breadth and, by inference, bulb
volume--within the range of our sample of diurnal anthropoids. Variations in the relative size of the olfactory bulbs in platyrrhines appear to relate
to the importance of olfaction in daily behaviors. Aotus has the largest olfactory bulbs among platyrrhines and relies more on olfactory cues when foraging than Cebus, Callicebus, or Saguinus (Bolen and Green, 1997; Bicca-Marques and Garber, 2004). As in other examples of nocturnal versus diurnal primates, nocturnality may have been the environmental factor that selected for this
difference in Aotus, (Barton et al., 1995), although communication and other
behaviors are also likely to select for olfactory variation in diurnal anthropoids.
Considering the olfactory fossa size of Tremacebus, olfactory ability of this
Miocene monkey was probably not as sensitive as in Aotus and counts against
the hypothesis that Tremacebus was nocturnal. This finding accords well with
previous observations that the orbits of Tremacebus are not as large as nocturnal Aotus (Kay and Kirk, 2000).

About the Species

This specimen, the holotye of Tremacebus harringtoni consists of a partial skull received by Carlos Rusconi in 1932 from Sr. Thomás Harrington who collected it from the Early Miocene (~22 MA, Colhuehuapian SALMA) from approximately 12 km southwest of Cerro Sacanana, in northcentral Chubut Province, Argentina.

Rusconi named the specimen as a species of Homunculus (Rusconi, 1933), and provided an extended description of the fossil (Rusconi, 1935). Philip Hershkovitz (1974) proposed a new genus for this skull. At some point after Rusconi’s original description, a substantial amount of plaster was added to reconstruct missing parts. Later, Hershkovitz and others made an effort to have the plaster removed but succeeded only partially because the plaster is cleverly tinted to match the color of the fossil bone. In an effort to determine more precisely the limits of bone, matrix, and plaster, and to better appreciate the structural details of the interior of the skull, Richard Kay borrowed the specimen from Dr. Jaime Powell, curator of the Rusconi Collection at Museo de Fundación Miguel Lillo, Tucuman, for CT scanning. Details of the preservation provided by Hershkovitz (1974) are substantially correct except that the apex of the orbit and optic foramen are not preserved as Hershkovitz claimed, and the lateral pterygoid plate figured by Hershkovitz consists of plaster. Hershkovitz’s claim that there was a large inferior orbital fissure in life cannot be confirmed.

About this Specimen

The specimen was scanned by Matthew Colbert on 12 November 2002 along the coronal axis for a total of 1177 slices, each slice 0.0466 mm thick, with an interslice spacing of 0.0466 mm.